It is now well recognized that the world is in the midst of a global epidemic of diabetes. Diabetes has become one of the leading causes of death and disability in the world (1). Patients can be at serious risk depending on the concentration and duration of blood glucose values: low levels (hypoglycaemia) for any length of time are associated with acute danger (brain damage, even death), while high levels (hyperglycaemia) have a chronic impact over days or years including heart and kidney failure, blindness and amputations (2).
For proper diabetes management, frequent monitoring of blood glucose is required (3). The current standard technique for self-monitoring of blood glucose requires a skin puncture to draw a small blood sample. However, the discomfort and pain of the procedure lead to poor compliance. Quick, reliable and pain-free testing are three highly desirable characteristics for patients.
Over the past two decades the pursuit of non-invasive methods of glucose monitoring has resulted in the development of a number of optical technologies (4,5). The near infrared (NIR) spectral range has been well explored because of the relatively low water absorption (4,6). However, the NIR glucose absorption bands are weak (overtone and combination bands) and overlapped with other blood constituents. Separation often requires sophisticated processing algorithms.
In comparison, the middle infrared (MIR) region is extremely useful for glucose identification. Of particular significance is the prominent absorption peak in the 8.5-10.5 μm band which is due to the carbon-oxygen-carbon bond in the pyran ring of glucose. This feature is peaked at ca. 9.7 μm, and is isolated from other interfering peaks in human blood (7-12). A major difficulty in the MIR is the intrinsic high-background absorption coefficient of water which tends to fully dominate the relatively low absorption of normal glucose concentration in human blood.
Recently, a new approach to the non-invasive detection of glucose was disclosed in US Patent Application Publication No. US20070213607. This new method, which is henceforth referred to as wavelength modulated differential photothermal radiometry (WM-DPTR), involves the measurement of glucose in a sample by the differential detection of a thermal emission produced by two incident optical beams. The differential detection is achieved by modulating the intensity of the two beams 180° out of phase, and selecting the wavelengths of the two beams to be on and off of the glucose absorption peak near a wavelength near 9.7 μm.
While this method has specifically been shown to successfully detect glucose with a sensitivity superior to other optical methods, it is desirable to further improve the sensitivity of the method. It would also be advantageous to adapt the method to the measurements of other types of samples.